Question about implementation of `std::io::Error::source`

help
1

I'm having issues implementing something and don't understand why the implementation of std::io::Error::source is implemented the way it is. Specifically:

    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
        match self.repr.data() {
            ErrorData::Os(..) => None,
            ErrorData::Simple(..) => None,
            ErrorData::SimpleMessage(..) => None,
            ErrorData::Custom(c) => c.error.source(),
        }
    }

why does source for std::io::ErrorKind::Other "skip a level" by returning c.error.source(). Seems like it should return Some(c.errer.as_ref())?

2

The Display and Debug impl also directly forward to the custom error. If you do io::Error::new(kind, my_error) you aren't creating a new error object which is caused by my_error. Rather the returned io::Error and my_error are one and the same error. This is similar to how anyhow::Error also forwards Error::source() to the respective impls of the error it wraps once you convert it to a Box<dyn Error>.

3

If you do io::Error::new(kind, my_error) you aren't creating a new error object which is caused by my_error .

But this is exactly what you are doing. Admittedly it's a thin wrapper but from the perspective of the type system you are creating a new object with a new type.

I've figured out that I can work around this by just calling io::Error::into_inner to get the inner error, but this is:

  1. Confusing and non-obvious
  2. Requires an owned value. If you have a reference (which is not Clone) then it can't be done at all
  3. Even with an owned value requires special casing in generic code
4

Think of this from the perspective of code which prints errors, following the source chain. If it didn't "skip a level", the error message in the io::Error would be printed twice.

5

The wrapper is irrelevant to the consumer of the error. It's just an implementation detail.